Thank you AirGraph for answering me soon, the article does not solve the problem that only exists using Catalina. I imported only files generated by FCPX in m4v into iBook Author. Author imports them, converts them, after a while they disappear leaving only the widget placeholder. I found, on this chat, other people with the same problem and what worries me, reading the following article,
https://itunespartner.apple.com/books/
Resources for authors...
is that the page dealing with "tools"
https://itunespartner.apple.com/books/tools
appoints only Pages, or the certified external parters. What's going on with iBookAuthor?
I produce eBooks and i am ready to change production software, if i need, but i wait for official news. I just contacted the Apple support by phone and they say: more or less, "we don't know". News are very much appreciated.
Best regards
Aldo